Integrated Solar Flow Battery

  • Posted on: 11 July 2019
  • By: yzhao277

Due to the intermittent nature of sunlight, practical round-trip solar energy utilization systems require both efficient solar energy conversion and inexpensive large-scale energy storage. For this purpose, photoelectrochemical (PEC) solar water splitting, as we discussed, could serve as a chemical approach to store solar energy in the chemical bond of hydrogen fuel. However, many scientific and technological challenges have so far prevented PEC water splitting from becoming a practical technology. Taking a different approach, we have developed a new type of integrated solar energy conversion and electrochemical storage devices, which we call “solar flow batteries (SFBs)1-3”, by integrating efficient solar semiconductors in aqueous electrolytes with redox flow batteries (RFBs)4 using the same pair of redox couples. Compared with separated solar energy conversion and electrochemical energy storage devices, combining the functions of separated devices into a single integrated device may represent a more compact and cost-effective approach for off-grid electrification. We have been working on three interconnected aspects for the SFB research: (1) understanding the operation mechanisms of SFB devices; (2) developing the design principles and modeling methods to maximize the performance of SFBs; (3) the demonstration and device optimization of SFBs.


Concept and working mechanisms of SFBs

As illustrated in Figure 1a, the general design for an integrated solar flow battery device consists of three electrodes, namely a photoelectrode, a cathode and an anode, typically made of inert carbon felt. In such integrated SFB devices, solar energy is absorbed by semiconductor photoelectrodes and the photoexcited carriers are collected at the semiconductor-liquid electrolyte interface to convert the redox couples to fully charge up the device (i.e. store the solar energy into the electrolytes). This process is basically the same as that in photoelectrosyntheic cell.5 Depends on the specific photoelectrodes used, the SFB can have either two photoelectrodes (two side illumination)1, 3 or one photoelectrode.2 When electricity is needed, the charged up redox couples will be discharged on the surface of carbon felt electrodes as one would do in the discharge of a normal redox flow battery (RFB) to generate the electricity. The electrodes are connected differently in energy delivery and storage mode: the cathode and anode are connected with an external load to discharge the SFB (Figure 1b), while the photocathode and photoanode are connected to allow solar-driven unassisted device recharge (Figure 1c). We can also operate this device just as a photovoltaic (PV) solar cell by cycling the redox couples between the photoelectrodes and the counter electrodes ­to directly extract the electricity out, which is how regenerative photoelectrochemical solar cells work (Figure 1d).

Figure 1. (a) General design scheme and (b-d) the three operation modes of an integrated SFB device.

Design principles and demonstrations of high performance SFBs.

We started with our first proof-of-concept for a monolithically integrated SFB by combining regenerative Si solar cells and all organic quinone-based RFBs.1 To evaluate the overall efficiency of the SFB device, we introduced a new figure of merit, solar-to-output electricity efficiency (SOEE), which is defined as:

where Edischarging is the output electrical energy delivered on demand after storage and Eillumination is the total solar energy input. Our first typical prototype integrated device showed an average SOEE of around 1.7% over ten cycles.1 A photo of this prototype SFB device is shown in Figure 2a.

Figure 2. Photos of (a) first generation SFB device; (b) second generation SFB device.


With comprehensive mechanism study and deeper understanding of the operation principles of SFBs, we developed a set of design principles for highly efficient integrated SFB devices. The most crucial idea behind these principles is that, with the available high performance solar cells and RFBs, the RFB cell voltage should be matched as close as possible to the maximum power point of the photoelectrode, which demands carefully chosen redox couples and photoelectrode materials (Figure 3a). Building on such rational design principles and novel device design, we successfully demonstrated a high performance SFB device (Figure 2b) using highly efficient and high photovoltage tandem III-V solar cells and high voltage 4-OH-TEMPO/MV RFBs (Figure 3b).  Enabled by high efficiency photoelectrode, properly matched redox couples, and carefully designed flow field design, a record SOEE of 14.1% has been achieved for the SFB (Figure 3c).2 These works received wide-spread interests from both the scientific news media and the general public, including a front page story in Wisconsin State Journal. More importantly, this study lays out the design pathway for us to achieve even higher SOEE, potentially using much less expensive solar cell materials.


Figure 3. (a) Voltage matching design principles for the highly efficient SFB device; (b) Cyclic voltammetry of 4-OH-TEMPO/MV used in this SFB, featuring a large voltage difference of 1.25 V; (c) representative record setting SOEE (14.1%) of the SFB device over 10 cycles.


We have further worked on improving the SFB device lifetime that has received much less attention than efficiency, which could be partially attributed to different types of challenges involved in achieving longer device lifetime. Generally, there are two major challenges preventing those devices from reaching long device lifetime: instability of redox couples and corrosion of photoelectrodes by electrolytes. In collaboration with Prof. Michael Aziz and Prof. Roy Gordon’s group at Harvard University who provided us with stable organic redox molecules,6 we carefully studied the device failing mechanisms and applied the understanding to design both the organic redox molecules and photoelectrodes for long lifetime SFBs. Our research efforts have successfully extended the continuous operation lifetime of Si photoelectrode based SFB device from 10 hours to longer than 200 hours (Figure 4).3 By employing the SFB design principles above and further the newly developed concept of instantaneous SOEE, we boosted the SOEE from the 1.7% for our first prototype more than three-fold to 5.4% in this SFB using the same silicon photoelectrodes.


Figure 4. (a) Scheme of the long lifetime SFB, illustrating the architectures and energy diagrams of the illuminated photoelectrodes in equilibrium with BTMAP-Vi and BTMAP-Fc redox couples; (b) Discharging capacity utilization and SOEE of the SFB device showing a stable cycling performance over 200 hours (100 cycles).  


Our work has paved the way for a practical new approach to harvesting, storing and utilizing the intermittent solar energy with unprecedented high energy conversion efficiency and energy storage density. The major focus of our ongoing research is investigating the critical metrics of SFBs and pushing the boundaries to make this new approach competitive for powering off-grid applications, by both further increasing the SOEE and lowering the cost. We are further exploring new concepts in electrochemical energy conversion and storage to address the general challenges of intermittency of the renewable energy sources. 


(1) Li, W. J.; Fu, H. C.; Li, L. S.; Caban-Acevedo, M.; He, J. H.; Jin, S. Integrated Photoelectrochemical Solar Energy Conversion and Organic Redox Flow Battery Devices. Angewandte Chemie-International Edition 2016, 55, 13104-13108.

(2) Li, W. J.; Fu, H. C.; Zhao, Y. Z.; He, J. H.; Jin, S. 14.1% Efficient Monolithically Integrated Solar Flow Battery. Chem 2018, 4, 2644-2657.

(3) Li, W.; Kerr, E.; Goulet, M.-A.; Fu, H.-C.; Zhao, Y.; Yang, Y.; Veyssal, A.; He, J.-H.; Gordon, R. G.; Aziz, M. J.; et al. A Long Lifetime Aqueous Organic Solar Flow Battery. Advanced Energy Materials 2019, 1900918.

(4) Wei, X.; Pan, W.; Duan, W.; Hollas, A.; Yang, Z.; Li, B.; Nie, Z.; Liu, J.; Reed, D.; Wang, W.; et al. Materials and Systems for Organic Redox Flow Batteries: Status and Challenges. ACS Energy Letters 2017, 2, 2187-2204.

(5) Jin, S. What Else Can Photoelectrochemical Solar Energy Conversion Do Besides Water Splitting and CO2 Reduction? ACS Energy Letters 2018, 3, 2610-2612.

(6) Beh, E. S.; De Porcellinis, D.; Gracia, R. L.; Xia, K. T.; Gordon, R. G.; Aziz, M. J. A Neutral pH Aqueous Organic–Organometallic Redox Flow Battery with Extremely High Capacity Retention. ACS Energy Letters 2017, 2, 639-644.

7) Wenjie Li, Jianghui Zheng, Bo Hu, Hui-Chun Fu, Maowei Hu, Atilla Veyssal, Yuzhou Zhao, Jr-Hau He, T. Leo Liu, Anita Ho-Baillie, and Song Jin. High-performance solar flow battery powered by a perovskite/silicon tandem solar cell. Nat. Mater. 2020.


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